Bright gold electroplating bath and method of electroplating bright gold

ABSTRACT

A bright gold electroplating bath comprising: an aqueous solution of an alkali metal gold cyanide; as an alloying agent, cobalt or nickel, in the form of a complex with at least aminoguanidine and a chelating phosphonic acid; and as conducting and buffering agents, one or more bath soluble phosphates.

United States Patent 1191 Bick et al. Dec. 24, 1974 [54] BRIGHT GOLD ELECTROPLATING BATH 2,702,271 2/1955 Spreter et al. 204 44 N METHOD ()F ELECTROPLATING 3,367,853 2/1968 Schumpelt 204/46 3,475,290 10/1969 Yamamura et a]. 204/43 G BRIGHT GOLD 3,672,969 6/1972 Nobel et a1. 204/43 G [75] Inventors: Maurice Bick, South Orange; Jean A Loch, Metuchen both of NJ FOREIGN PATENTS OR APPLICATIONS I 1,909,144 9/1970 Germany 204/46 [73] Ass1gnee: Aur1c Corporation, Newark, NJ. 1,102,179 2/1968 Great Britain 204/46 [22] Flled: 1973 Primary Examiner-G. L. Kaplan [21] Appl. No.2 354,682 Attorney, Agent, or Firm-Stefan J. Klauber Related [1.8. Application Data [57] ABSTRACT [63] Continuation-impart of 176263 A bright gold electroplating bath comprising: an aque- 1971. abandoned.

ous solutlon of an alkah metal gold cyanide; as an al' [52] US. Cl. 204/43 G, 204/46 G I ying agent obalt or nickel in the form of a om- 1 1 (3243b C231) 31 plex with at least aminoguanidine and a chelating [58] Field of Search 204/43 G, 46, 44 phosphonic acid; and as conducting and buffering agents, one or more bath soluble phosphates. [56] References Cted 10 Claims, N0 Drawings UNITED STATES PATENTS 1 2,383,798 10/1945 Hull 204/50 Y 2,469,727 5/1949 Hoffman I. 204/49 BRIGHT GOLD ELECTROPLATING BATH AND METHOD OF ELECTROPLATING BRIGHT GOLD This application is a continuation-in-part of our copending application, Ser. No. 176,263, filed Aug. 20, 1971, and now abandoned for GOLD AND GOLD ALLOYS ELECTROPLATING BATH," which appli cation is assigned to the same assignee as is the instant application.

BACKGROUND OF THE INVENTION This invention relates generally to electroplating compositions and methodology, and more specifically relates to compositions and methods useful in the electroplating of bright gold.

Within recent years an increasing demand for high quality bright gold plating, e.g. for decorative and electronic applications, has prompted considerable industrial activity directed toward development of electro' plating baths useful for such purposes. Significant contributions to the art of electroplating bright gold alloys are thus, for example, disclosed in US. Pat. Nos. 2,812,200 and 2,905,601. The first of these patents to F. Volk, discloses a phosphate-based bath operating in the substantially neutral pH range of 6.5 to 7.5. The alloying metal or metals in this bath are introduced into the composition in the form of double cyanides, such as sodium copper cyanide or potassium nickel cyanide. The said baths, which produce bright, aesthetically pleasing deposits, have been of significant value, particularly in countries such as Switzerland and Germany, where the production of thick, low carat, gold deposits in the to 20 micron range, are much in demand, especially for the watch industry; and the baths are still widely used today for such applications. A principal disadvantage, however, of such baths are their requirement for relatively high plating temperatures (60 to 75C) and their rather short life. In the latter respect it may be observed that such baths require changing upon the initial gold content being turned over four or five times.

Yet, a further disadvantage of these conventional phosphate-based baths is a tendency thereof to pro duce pitting in the resulting plating. Although the rea son for this is not completely understood, it is hypothesized that undesirably high surface tension effects bubbling, and so forth, during the deposition process, which may induce the specified defects.

The cited, and more recent, US. Pat. No. 2,905,601 to Rinker et al, discloses the use of weak stable organic acids, principally citric acid, in alkali metal cyanide baths, employing cobalt, nickel, and indium as bright eners, the said baths being maintained in the pH range of 3 to 5. A principal advantage of these latter baths is one of providing a stable electrolyte with a long life, which yields good, bright deposits.

A principal teaching of the Rinker bath is the use of citric acid or other weak organic acids in alloy gold plating baths. It appears here that the citric acid, or other weak organic acid, acts in the acid gold electroplating baths, as to tend to slow down the rate of deposit of such metals as nickel, copper, and cobalt. A teaching of similar nature is also set forth in an earlier patent, namely, US. Pat. No. 1,924,439 to H. Kersten. However, a serious deficiency of the said baths is that the resulting deposits are often highly stressed, and as a practical matter are often limited for such reason to thicknesses below about 5 microns. This detrimental effect probably is caused (at least in part) by hydrogen embrittlement and from the presence of organic decomposition products in the bath. It may be noted in the latter connection, that in such baths the rate of con sumption of citric acid is about equal to the weight of gold deposited.

In principle, it would appear that a bright gold electroplating bath possessing the advantages set forth with respect to the phosphate-based formulations of Volk, and the stability achieved in the Rinker formulations, would be much desired were the cited undesirable features of the said baths not present. Up to the present time, however, a bath satisfying such criteria has not been in evidence.

In accordance with the foregoing, it may be regarded as an object of the present invention, to provide electroplating baths useful in the plating of bright gold, which, while utilizing phosphates therein, possess a high degree of stability.

It is a further object of the present invention, to provide electroplating baths and methodology useful in the plating of bright gold, which are capable of yielding smooth, lustrous, high quality plating, even in thicknesses exceeding 5 microns.

It is a yet further object of the present invention, to provide electroplating baths useful in the plating of bright gold, which, while possessing the stability of citrate-based baths, do not require constant addition of salts to maintain bath conductivity and workability, as is characteristic of the citrate baths; and which further more yield the comparatively brighter deposits that are characteristic of phosphate-based baths.

SUMMARY OF THE INVENTION Now in accordance with the present invention, the foregoing objects, and others as will become apparent in the course of the ensuing specification, are achieved in a phosphate-based bright gold electroplating bath, wherein the cobalt or nickel is present in the form of a complex of specified phosphonic acids with aminoguanidine. By incorporating the base metal in the indicated complex form, the prior art disadvantages of the phosphate baths are overcome, in consequence of which the baths of the invention exhibit excellent stability and yield high quality platings, devoid of pitting, surface irregularities or the like, and possessing low porosity, and good grain and color, even in platings of relatively high thickness. The additive aminoquanidine appears to function somewhat in the present environ ment in an analogous way to the function of the weak organic acids in the cited U.S. Pat. Nos. 1,924,439 and 2,905,601. In some manner, not completely understood at the present time, a synergistic action appears to occur between the phosphonic acids of the invention and aminoguanidine, with the action of the resultant combination being such as to in part enable the results achieved in the invention.

Phosphates are present in the baths as the ammonium or alkali metal salts, and serve as conducting and buffering agents. In preparing the baths of theinvention one adds the conducting and buffering compounds in quantities which yield a bath specific gravity appropriate to the bath application. Thereupon the pH of the bath, as required, may be adjusted by addition of small quantities of innocuous acids or bases as is known in the art. A useful specific gravity range for the baths is between about 6 to 20 Eaume, and a preferred range between about 10 and 16 Baume.

The aminoguanidine, which is initially added in the form of the bicarbonate (CI-I N .I-I CO available egg. from Eastman Organic Chemicals of Rochester, New York, is typically present in the bath in concentrations of from about 2 to 50 g/l, and preferably in the range of about 5 to 35 g/l, calculated as the bicarbonate. Gold may be present in the baths in concentrations of from about 2 g/l to saturation. The alloying cobalt and/or nickel is typically present in concentrations of from about 5 mg/l to about 40 g/l, calculated as the metal, with a preferred level being between about mg/l and 5 g/l, calculated as the metal.

DESCRIPTION OF PREFERRED EMBODIMENT In order to illustrate the advantages of the unique combination of the present invention, an electroplating bath is firstly set forth in Example I below, which is basically a prior art phosphate-based bath; and in Example II a bath is set forth utilizing a combination resembling in some respects the baths of the present invention, but devoid of the aminoguanidine which is present (in a complex) in the baths of the invention:

In each instance in the above, the cobalt and gold are calculated as the metal. Test panels prepared by electroplating from each of the baths of Examples I and 11 displayed a bright deposit, which, however, was not uniform in color. Some areas of the said panels were richer in gold coloration than other areas, it appearing that the gold tended to deposit in strongly agitated areas.

For purposes of the ensuing Examples, a cobalt complex was prepared as follows:

Example Ill Amino trimethylenephosphonic 140 ml acid (ATMPA) Aminoguanidine bicarbonate 50 g Cobalt carbonate (45.1% cobalt metal) 440g Deionized water to yield 100 ml In a typical procedure for preparing the said cobalt complex, the cited phosphonic acid (which is available commercially from Monsanto under the designation Dequest 2000) is diluted, and the cobalt carbonate slowly added. Thereupon the resulting mixture is warmed to completely dissolve the cobalt carbonate. The aminoguanidine carbonate is thereafter added very slowly, the resulting admixture filtered, if necessary,

and the resultant pH adjusted to about 3.6 (typically from about 3.0 to 4.0). Each mililiter of the resultant concentrate contains approximately 20 mg of cobalt metal.

The corresponding nickel-based complex is prepared in accordance with the following composition:

Example IV Ethylenediarnine tetra methylene 200 grams bphosphonic acid (EDMPA) icke carbonate (48.5% nickel 41.2 grams metal) Aminoguanidine 50 grams Deionized water to yield 1000 ml dissolved, then the pH is adjusted, if necessary, with"""= EDMPA or aminoguanidine to 3.0 to 5.0. As appropriate, filtering is then effected. Each ml of this solution contains approximately 20 mg of nickel as the metal.

EXAMPLE V For purposes of this Example, the same bath as indicated in connection with Example II is utilized, except that in this instance the cobalt complex with aminoguanidine which is set forth in Example 111 was employed. More specifically 10 g/l of aminoguanidine was now present in the said electroplating bath. Coloration of the bath, in consequence, was completely distinct from that of Example 11, indicating that a change had occurred in the complex used in Example 11. Utilizing the bath of the present Example, panels plated under the substantially the same conditions as in Example II, were found to yield highly acceptable deposits superior in all respects to those of Examples I and II, but with the most marked improvement being in the uniformity of the resulting plating.

The phosphonic acid compound noted above, is one of a group of chelating phosphonic acids that are generally suitable for the present applications. The compound indicated, which may for purposes of convenience be referred to as ATMPA, is the prefnabm agent for use where cobalt is to be alloyed with the gold. The cited compound is not as effective where nickel is to be plated with gold, as is another wellknown of the said chelating phosphonic acids, namely, ethylenediamine tetra methylene phosphonic acid (EDMPA). This latter compound is also available from Monsanto, under the designation Dequest 2041." In preparing the complexes of Examples III and IV, the chelating phosphonic acid is added on approximately a 1:1 mole ratio with the nickel or cobalt although an excess of the chelating agent is not objectionable.

The stability of the baths in acccordance with the invention can be increased further by the addition of from about 1 to 50 g/l of boric acid (preferably from about 10 to 30g/l) and from about 1 to 30 g/l of glycerine, as is described in British Pat. No. 1,218,732. In some instances the baths of the present invention still have a tendency to produce the insoluble compounds which occur where baths of the type set forth in Example I are utilized, such compounds being detrimental to the good performance of the bath. Consequently, prea complex with at least aminoguanidine and a chelating phosphonic acid; the concentration of said aminoguanidine being from about 2 g/l to 50 g/l,

Example VI and said phosphonic acid being present in suff- Gold metal B g/ 5 cient uantity to complex saidl alloying agent, Ammonium phosphate monobasic 150 g) as additional conducting and buffering agents, one or Cobalt, as the complex of more bath soluble phosphates; 52 235 the pH of said bath being between 3 and 6, and the Glycerine 10 mm concentration of said conducting and buffering pH (adjusted by Nl-hOH or 10 agents being appropriate in the presence of the reg gggfiz to make $8 55 maining bath components to adjust the bath spe cific gravity to a desired level of from about 6 up to 20 Baume, said level being above that which The bath of the above Example VI operates very effecwould be achiefved in h absence of m tively at 37C, and a current density up to 1.2 am /d z 2. An electroplating in accordance with claim 1, The ammonium salts used in this bath are the preferred Wherem mamlamed betwea'n about salts for the present compositions, and are typically and the Speclfi? gravlttbetween to m l present in the range of about 30 to 225 g/l, yielding a 3. An electroplating bath in accordancewith claim 2, specific gravity of at least 0 Baume. Their function wherein said aminoguanidine is present in concentrapears to be one of augmenting the stability of the baths. of from aboul 5 to I The preferred pH for both the nickel and cobalt baths ele ctroplatmg h m accordance Clalm is between 33 and 4.3 but the acceptable range is wherein said phosphates include from about to 225 more generally between 3 and 6. At a pH of3.8 the effiof ammonium phosphoate monobaslc to yleld a Speciency of the bath of Example Vi was found to be as Clfic gravlty of at E 8 follows: 25 5. An electroplating bath in accordance with claim 4, further including as additional stabilizers from about 1 to 50 g/l of boric acid, and from about 1 to 30 g/l of 50C 44.8 mg/AM glycerine.

6. An electroplating bath in accordance with claim 1, 30 wherein said alloying agent comprises between 10 mg/l C 48- and 5 grams/l of cobalt, and said phosphonic acid com prises amino-trimethylenephosphonic acid. Further variations of the bath set forth in Example Vl 7. An electroplating bath in accordance with claim 1, are indicated below in Examples Vll through IX: wherein said alloying agent comprises between 10 mg/l Examples Vll VIII IX Gold 1 8 g/l 8 g/l 8 g/l Ammonium phosphate monobasic l20 g/l 120 g/l 120 g/l Boric acid 20 g/] 20 g/l 20 g/l Glycerine l0 ml/l l0 ml/l l0 l'lll/l Aminoguanidine bicarbonate Sgg/I 5 g/l g/l Cobalt (as the complex of Example ill) I 1 l0() mg/l Nickel (as the complex of Example IV) 200 mg pH 4.2 4,2 4.3

The efficiency observed during plating with bath Vll and 5 grams/l of nickel, and said phosphonic, acid comwas 35 mg/AM at 30C; and 34 mg/AM at 40C. The prises ethylenediamine tetra methylenephosphonic efficiency for bath Vlll was 42 nig/AM at 37C; and the acid. efficincy of bath X was 2 lg/AM F 50 8. A method for electroplating bright deposits of gold While the present mention been partlculatly l alloyed with a metal selected from the group consisting forth in terms of specific embodiments thereof, it will of Cobalt and nickel, comprising, be understood in view of theinstant disclosure, that nupreparing a Complex of a metal selected from the memus vaflatlofls "P the P P l enabled group consisting of cobalt and nickel, with at least to those skilledin the art, which variations in propriety, aminoguanidine and a Chelating phosphonic acid; yet reside within the true scope of the instant teaching. preparing an aqueous electroplating bath of an alkali Accordingly, the invention is to be broadly construed, meta! gold Cyanide, together with Said COmPleX, and limited only by the scope and spirit of the claims and as ccmducting and buffering agents one or now appernded heretomore bath soluble phosphates; the pH of said bath We Claim: being between 3 and 6, and said gold being present A bright gold electroplating bath, C0mPl'l5mg3 in concentrations of from about 2 g/l to saturation, aqueous Solution of an alkali metal gold cyanide, said complex including in said bath from about 4 said gold being present in concentrations of from mg/l to 40 g/l of said alloying metal; the concentraabout 2 g/l to saturation; tion of said aminoguanidine being from about 2 g/l between 4 mg/l and 40 grams/l calculated as the to 50 g/l, and said phosphonic acid being present metal, of an alloying agent selected from one or more members of the group consisting of cobalt and nickel, said agent being added to said bath as in sufficient quantity to complex said alloying metal; and the concentration of said conducting and buffering agents being appropriate in the prespH of said bath is maintained between about 3.8 and 4.3, and the specific gravity between 10 and 16 Baume.

10. A method in accordance with claim 9, wherein as further stabilizers, between about 1 to 50 g/l of boric acid, and from about 1 to 30 g/l of glycerine, are pres- 

1. A BRIGHT GOLD ELECTROPLATING BATH, COMPRISING: AN AQUEOUS SOLUTION OF AN ALKALI METAL GOLD CYANIDE, SAID GOLD BEING PRESENT IN CONCENTRATIONS OF FROM ABOUT 2 G/L TO SATURATION: BETWEEN 4 MG/ AND 40 GRAMS/L CALCULATED AS THE METAL, OF AN ALLOYING AGENT SELECTED FROM ONE OR MORE MEMBERS OF THE GROUP CONSISTING OF COBALT AND NICKEL, SAID AGENT BEING ADDED TO SAID BATH AS A COMPLEX WITH AT LEAST AMINOGUANIDINE AND CHELATING PHOSPHONIC ACID; THE CONCENTRATION OF SAID AMINOGUANIDINE BEING FROM ABOUT 2 G/L TO 50 G/L. AND SAID PHOSPHONIC ACID BEING PRESENT IN SUFFICIENT QUANTITY TO COMPLEX SAID ALLOYING AGENT; AS ADDITIONAL CONDUCTING AND BUFFERING AGENTS, ONE OR MORE BATH SOLUBLE PHOSPHATES; THE PH OF SAID BATH BEING BETWEEN 3 AND 6, AND THE CONCENTRATION OF SAID CONDUCTING AND BUFFERING AGENTS BEING APROPRIATE IN THE PRESENCE OF THE REMAINING BATH COMPONENTS TO ADJUST THE BATH SPECIFIC GRAVITY TO A DESIRED LEVEL OF FROM ABOUT 6* TO UP TO 20* BAUME, SAID LEVEL BEING ABOVE THAT WHICH WOULD BE ACHIEVED IN THE ABSENCE OF SAID AGENTS.
 2. An electroplating bath in accordance with claim 1, wherein said pH is maintained between about 3.8 and 4.3, and the specific gravity between 10* to 16* Baume.
 3. An electroplating bath in accordance with claim 2, wherein said aminoguanidine is present in concentrations of from about 5 to 25 g/l.
 4. An electroplating bath in accordance with claim 1, wherein said phosphates include from about 30 to 225 g/l of ammonium phosphate monobasic, to yield a specific gravity of at least 8* Baume.
 5. An electroplating bath in accordance with claim 4, further including as additional stabilizers from about 1 to 50 g/l of boric acid, and from about 1 to 30 g/l of glycerine.
 6. An electroplating bath in accordance with claim 1, wherein said alloying agent comprises between 10 mg/l and 5 grams/l of cobalt, and said phosphonic acid comprises amino-trimethylenephosphonic acid.
 7. An electroplating bath in accordance with claim 1, wherein said alloying agent comprises between 10 mg/l and 5 grams/l of nickel, and said phosphonic acid comprises ethylenediamine tetra methylenephosphonic acid.
 8. A method for electroplating bright deposits of gold alloyed with a metal selected from the group consisting of cobalt and nickel, comprising: preparing a complex of a metal selected from the group consisting of cobalt and nickel, with at least aminoguanidine and a chelating phosphonic acid; preparing an aqueous electroplating bath of an alkali metal gold cyanide, together with said complex, and as conducting and buffering agents one or more bath soluble phosphates; the pH of said bath being between 3 and 6, and said gold being present in concentrations of from about 2 g/l to saturation, said complEx including in said bath from about 4 mg/l to 40 g/l of said alloying metal; the concentration of said aminoguanidine being from about 2 g/l to 50 g/l, and said phosphonic acid being present in sufficient quantity to complex said alloying metal; and the concentration of said conducting and buffering agents being appropriate in the presence of the remaining bath components to adjust the bath specific gravity to a desired level of from about 6* up to 20* Baume, said level being above that which would be achieved in the absence of said agents; and electrolyzing said aqueous bath to yield the desired electroplating.
 9. A method in accordance with claim 8, wherein the pH of said bath is maintained between about 3.8 and 4.3, and the specific gravity between 10* and 16* Baume.
 10. A method in accordance with claim 9, wherein as further stabilizers, between about 1 to 50 g/l of boric acid, and from about 1 to 30 g/l of glycerine, are present in said bath. 